Scanning optical delay device having a helicoid reflecting mirror

ABSTRACT

A rapid and linear scanning optical delay line was attained by the use of a helicoid reflecting mirror which was spun by a DC servo motor for bringing about a periodic change in the optical path length of the reflected light beam. The double-pass scanning range of the delay time as large as 80 picosecond was attained by the scanning optical delay line having a helicoid reflecting mirror having 12 mm in pitch distance. The scanning optical delay line was used in an optical second-harmonic generation autocorrelator, which was thus capable of scanning successfully a picosecond optical pulse train. A real-time scanning of the picosecond laser pulse of a mode-locked Titanium:Sapphire was verified.

This application is a divisional application of U.S. application Ser.No. 08/760,036, filed on Dec. 4, 1996, now U.S. Pat. No. 5,784,186.

FIELD OF THE INVENTION

The present invention relates generally to a scanning optical delaydevice, and more particularly to a scanning optical delay device havinga helicoid reflecting mirror capable of a rapid and linear scanningoptical delay.

BACKGROUND OF THE INVENTION

The fast and large-scale scanning optical delay device was employed inthe noncollinear autocorrelator by Wang and Pan in 1995. Chi-Luen Wangand Ci-Ling Pan, Opt. Lett, 20, 1292 (1995)!. Similarly, it was employedin the pump probe experiment by Ganikhanov, et. el. in 1995. FeruzGanikhanov, Gong-Ru Lin, Wen-Chungg, C-S Chang, and Ci-Ling Pan, 67,3465 (1995)!. The current application of the scanning optical delaydevice involves the incorporation of a shaker R.F. Fork and F. A.Beissoer, Appl. Opt. 3534 (1978)! or a pair of rotatable parallelmirrors Z. A. Yasa and N. M. Amer, Opt. Commun. 36, 406 (1981)! into anautocorrelator for displaying the autocorrelation signals of laserpulses by an oscilloscope. In addition, the optical delay of scanningspeed as high as several hundred Hertz can be attained by a combinationof optical grating, lens and vibrating mirror. For more details, pleaserefer to Z. A. Yasa and N. M. Amer, Opt. Commun. 36, 406 (1981). Suchdevices as described above are limited in design in that they arecapable of attaining the scanning range of the delay time for only a fewpicosecond, and that they can not be applied to a pump probe experimentin which the scanning of the wider laser pulse or the longer responsivetime is called for. Moreover, the photoelectric measurement of the Sparameter of transistor requires a longer optical delay time so as toattain with precision a low frequency response of the element, as shownby K. F. Kwong, D. Yankelevich, K. C. Chu, J. P. Heritage, and A.Dienes, Opt. Lett. 18, 558 (1993). The combination of a cam and a sliderail is capable of generating a scanning optical delay of 300picosecond, as shown by D. C. Edelstein, R. B. Romney, and M.Scheuermann, Rev. Sci. Instrum. 62, 579 (1991). By using the combinationof rotatable prisms, a fast scanning autocorrelator capable of ascanning range of the delay time as high as one nanosecond isattainable, as shown by G. Xinan, M. Lambsdorff, J. Kuhl, and E.Biachang, Rev. Sci. Instrum. 59, 2088 (1988).

SUMMARY OF THE INVENTION

The present invention discloses a scanning optical delay devicecomprising:

a helicoid reflecting mirror having a rotating shaft and a spiral strapwound on said rotating shaft along the direction of an axis of saidrotating shaft, said spiral strap having thereon a smooth surfacecapable of reflecting partially or entirely an incident light beam,wherein any point of said smooth surface has a position capable of beingexpressed by a column coordinate equation as follows:

    2πz-dφ=0

in which z is a coordinate along the direction of

said axis of said rotating shaft; d, a pitch of said spiral strap; andφ,

an angular coordinate encircling said axis of said rotating shaft; and

a rotating mechanism for driving said helicoid reflecting mirror torotate around said axis of said rotating shaft;

wherein said any point of said smooth surface is capable of bringingabout a reflected light beam having a variable optical path length atthe time when an incident light beam strikes said any point of saidsmooth surface provided that said incident light beam is parallel tosaid axis of said rotating shaft, said variable optical path length ofsaid reflected light beam being subject to a continuous change at suchtime when said helicoid reflecting mirror is driven by said rotatingmechanism to rotate around said axis of said rotating shaft.

Preferably, said smooth surface of said spiral strap has a smoothnessenabling said smooth surface to reflect almost entirely said incidentlight beam.

Preferably, said smooth surface is capable of reflecting a light beam ofa wave length ranging between ultraviolet ray and infrared ray.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a helicoid reflecting mirror of afirst preferred embodiment of the present invention.

FIG. 2 is a time vs. optical path length difference plot, which shows avariation of optical path length difference of a scanning optical delaydevice having the helicoid reflecting mirror of the first preferredembodiment of the present invention.

FIG. 3 shows a schematic view of an optical second-harmonic generationautocorrelator of the present invention.

FIG. 4 shows a periodic scanning laser pulse autocorrelation signaldisplayed by an oscilloscope of the optical second-harmonic generationautocorrelator as shown in FIG. 3.

FIG. 5 is similar to FIG. 4 except that the time axis of FIG. 5 isexpanded.

FIG. 6 shows a perspective view of a helicoid reflecting mirror of asecond preferred embodiment of the present invention.

FIG. 7 shows a perspective view of a helicoid reflecting mirror of athird preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

As illustrated in FIG. 1, the helicoid reflecting mirror of the firstpreferred embodiment of the present invention is capable of attaining ascanning range of optical delay line as large as 80 picosecond and isincorporated into a second-harmonic generation nonlinear autocorrelator,which is thus capable of a successful scanning of the laser pulseautocorrelation signal at the picosecond level.

The mathematical equation of the helicoid reflecting mirror of the firstpreferred embodiment of the present invention can be expressed by acolumn coordinate as follows:

    2πz-dφ=0                                            (B 1)

In the above equation, z and φ stand respectively for the values of thecoordinate axes along the rotating axis and the rotating direction,whereas d stands for the pitch. The normal line perpendicular to themirror surface can be expressed by a mathematical equation as follows:##EQU1##

As long as the direction of incidence beam is parallel to the rotatingaxis (z-axis), the radius ρ of the reflecting point of the incidencebeam on the mirror in motion remains constant. On the basis of theequation (2), it is apparent that the direction of the normal line ofthe reflection point also remains unchanged. In other words, thedirection of reflection beam will remains constant with a minute changeof optical path length. If the reflection beam and a split beam arefocused by a lens, the light beams can be converged at a common point onthe focal plane.

The method for making the helicoid reflecting mirror of the firstpreferred embodiment of the present invention involves a first step ofmaking a cylindrical body of aluminum, and then machining thecylindrical body of aluminum the same way as forming square threads. Thecylindrical body is then ground and polished with the sand papers ofvarious roughness until the surface of the cylindrical body is smoothand glossy in conformity with the requirements of the reflecting mirror.The reflecting mirror has a diameter (D) of 50 mm, a pitch (d) of 12 mm,and a depth (T) of 15 mm as shown in FIG. 1. The rotating shaft of thereflecting mirror is fastened with a DC servo motor (not shown in thedrawings) so as to enable the reflecting mirror to rotate continuously.As a light ray strikes the reflecting surface of the reflecting mirrorsuch that the light ray is parallel to the rotating axis, the light rayis reflected. The optical path length of the reflected light is changedcontinuously and linearly in view of the fact that the reflectingsurface of the helicoid reflecting mirror is turned continuously. Thevariation of the optical path length is shown in FIG. 2. The period isdependent on the revolving speed of the motor, whereas the total opticalpath length difference is dependent on the pitch. The scanning range ofthe delay time of the first preferred embodiment of the presentinvention is 80 picosecond for the helicoid reflecting mirror having apitch of 12 mm. Accordingly, it is necessary to increase the pitch up to150 mm so as to attain the total optical path length difference at ananosecond level. Without increasing the pitch, a plurality of thehelicoid reflecting mirrors of the first preferred embodiment of thepresent invention must be used such that the light ray is reflected backand forth for several times.

In order to prove the validity of the optical delay line of the firstpreferred embodiment of the present invention, the helicoid reflectingmirror was used in the second-harmonic generation autocorrelator, asshown in FIG. 3. After a light ray had entered the autocorrelator, thelight ray was split by a beam splitter 10 into two light beams, one ofwhich was reflected by the helicoid reflecting mirror 20 of the firstpreferred embodiment of the present invention and the other beam, i.e.,the "split beam", is reflected by a first reflector 11 that reflects thesplit beam along a predetermined path toward a lens 30. The position ofthe mirror 20 was adjusted with precision so as to keep the reflectedbeam on the optical plane of the entire system. The reflecting spot wasadjacent to the fringe of the mirror 20 for minimizing the deformationof the reflected light. In view of the fact that normal line of themirror 20 had a component of φ direction, an angle θ was formed betweenthe reflected beam and the incident beam. The angle θ was expressed bythe following equation (3): ##EQU2##

A second reflector 12 redirects the reflected light beam from thehelicoid reflecting mirror along a path substantially parallel to thepath of the split beam reflected from the first reflector 11. Thereflected beam and the split beam were converged by a lens 30. The focalpoint was provided with a nonlinear second-harmonic generation crystal40 for generating the second-harmonic optical signal to be measured. Theoptical signal was filtered by a filter 50 before the optical signal wasreceived by a photomultiplier tube (PMT) 60. The second-harmonic opticalsignal was then converted into an electrical signal by PMT 60. Theelectrical signal was received by an oscilloscope 70 through which thescanned autocorrelation signal was displayed.

An optical pulse of 1.7 picosecond was introduced into thesecond-harmonic generation autocorrelator as a source of light tubemeasured. As the helicoid reflecting mirror 20 of the first preferredembodiment of the present invention was rotated, a periodic scannedsignal was displayed by the oscilloscope 70, as shown in FIG. 4. Theperiodic scanned signal was also represented in FIG. 5 in which the timeaxis was expanded. According to FIGS. 4 and 5, it was readily apparentthat the scanning frequency of the second-harmonic generationautocorrelator was as high as 43.5 Hz, and that the pulse width wasscanned with precision. The autocorrelation signal was slightly frombeing symmetrical in view of the fact that the stability of the rotatingshaft of the motor used in the first preferred embodiment of the presentinvention was poor. The poor stability of the rotating shaft of themotor was responsible for the swaying of the helicoid reflecting mirrorin motion. Such a problem as described above can be easily resolved byusing a stable motor.

The resolution of the linearly-scanned optical delay time can beexpressed by a mathematical equation (4) as follows: ##EQU3##

In the above equation (4), w stands for the width of the light beam,whereas c stands for the light speed in the vacuum. According to thefirst preferred embodiment of the present invention, the resolution wasabout 0.5 picosecond. The resolution was limited mainly by the width(about 1 mm) of the light beam and the ratio (d/ρ) of the pitch of thehelicoid reflecting mirror and the radius of the reflecting point. It issuggested that a resolution of 10 femtosecond is attainable if the pitchis reduced to an extent that the pitch is about one twentieth of thesize of the pitch of the first preferred embodiment of the presentinvention. However, a reduction in the pitch of the helicoid reflectingmirror can result in a reduction in the total delay time (2 d/c). Itmust be added here that the resolution of 10 femtosecond is alsoattainable if the radius of the helicoid reflecting mirror is made aslarge as possible.

The embodiment of the present invention described above is to beregarded in all respects as being merely illustrative and notrestrictive. Accordingly, the present invention may be embodied in otherspecific forms without deviating from the spirit thereof. For example,the helicoid reflecting mirror of the first preferred embodimentdescribed above may be provided with a plurality of the interconnectedreflecting planar mirrors in place of the spiral curved surface, asillustrated in FIG. 6. The surface of each of the reflecting planarmirrors has a normal line perpendicular to the direction of the rotatingshaft radius. In addition, the angles formed by the normal lines and theaxis of the rotating shaft are different from one another. Moreover, thehelicoid reflecting mirror of the first preferred embodiment of thepresent invention may be provided with a plurality of the interconnectedconcave mirrors (as shown in FIG. 7) or convex mirrors (not shown in thedrawing) in place of the spiral curved surface of the first preferredembodiment. Each of the concave or convex mirrors has an opticalsymmetrical axis perpendicular to the direction of the rotating shaftradius. In addition, the angles formed by the optical symmetrical axesand the axis of the rotating shaft are different from one another.Moreover, the helicoid reflecting mirror of the present invention may beprovided with a combination of the spiral curved surface, theinterconnected planar reflecting mirrors, the interconnected concavemirrors and the interconnected convex mirrors.

The validity of the present invention was positively confirmed by theexperimental results as described above. The present invention can beused in the autocorrelator capable of scanning successfully theautocorrelation curve of the picosecond optical pulse. The scanningoptical delay device of the present invention has therefore a great dealof potential in the research and the commercial applications.

What is claimed is:
 1. A scanning optical delay device comprising:ahelicoid reflecting mirror having a rotating shaft and a reflectingpseudo-helicoid strap wound on said rotating shaft along the directionof an axis of said rotating shaft, said reflecting pseudo-helicoid straphaving a plurality of concave or convex reflecting mirrorsinterconnected such that an optical symmetrical axis of each of saidconcave or convex reflecting mirrors is perpendicular to the directionof a radius of said rotating shaft, and that angles formed between saidoptical symmetrical axes and said axis are different from one another.